PFC is often used in AC/DC conversion, in order to draw current sinusoidally in phase with a sinusoidal voltage. Ensuring adequate dynamic range of a PFC implementation, so that it can operate properly over a wide range of loads from light load to heavy load, can be an issue.
A particular architecture that can be used for PFC is one that does not use input voltage sensing and is sometimes called a “Doff controller”, an “Indirect PFC Controller”, or a “Single-Cycle PFC Controller” (although the latter example is a specific type of implementation that can be used with Doff/Indirect controllers). Doff is the portion or percentage of time that a boost switch in a power converter is off, i.e., it is 1-Don, where Don is the portion or percentage that a switching signal causes the switch to be on.
This type of PFC is based on the fundamental concept that Doff=Vin/Vout. Since Vout is a constant DC voltage and Vin is a sinusoidal input voltage in the case of AC/DC conversion, Doff must vary sinusoidally over the input voltage waveform. PWM (Pulse Width Modulation) can be used where a voltage across an input current sensing resistor is compared to a frequency ramp, for example. In order for Doff to vary sinusoidally, the current sensing voltage would also need to vary sinusoidally in phase with Vin, and reach a high enough value at its maximum so that the condition Doff=Vin(peak)/Vout is satisfied. This must also occur independently of the load, and independently of the level of input current that the PFC is sensing.
These conditions can require very high gains if the sensed input current is very small, i.e., under light load. The required gain will also be higher for high AC input voltages, such as 240V or 265V as opposed to 120V, since Doff must now reach a larger value, at the same time as the sensed input current is actually lower under the same power load. For an 85Vrms (120V peak) input and 400VDC output for instance, Doff must reach about 30%, whereas for a 265Vrms (375V peak) input and 400V output, Doff must reach about 94%. In the latter case, the sensed input current would be only about 85/265=0.32 the current in the former case, thus requiring a much higher gain on the current sensing voltage to arrive at the required Doff.
Where an implementation has a finite maximum gain, there are some low values of input current where it may be impossible to multiply up enough to achieve the desired Doff value. This could be exacerbated at high input line voltages. Under this condition, the PFC might keep the boost switch on for a longer time than desired, increasing the current into the load and causing an over-voltage condition. This may result in a hysteretic behaviour or pulse-skipping, neither of which maintains the desired sinusoidal behaviour, impacting the Power Factor (PF) and Harmonic Distortion (HD) and possibly causing acoustic noise due to transformer mechanical noise. Another possible effect is stress on components such as capacitors. For example, if a converter goes out of regulation and into an over-voltage condition, higher voltages might be applied to hold-up capacitors, which also is not desirable.